The role (and even the number) of the early region functions involved when SV40 transforms a cell is unresolved and complex. Efficient transformation requires small t; yet transformation can occur at low frequency without it. Large T is also required in some clones to maintain the transformed state; yet in other cases it is not required. We have begun to analyze a new mutant in the early region with novel effects on transformation not predicted by known large T or small t effects. We plan to continue the in-depth analysis of these three functional types of mutants and their interactions by clonal analysis, protein analysis, copy number and rescue and testing of transforming genomes. We will attempt to generate a ts small t mutant which should then provide us with conditional mutants for all three SV40 functions which have been implicated in viral transformation and a unique opportunity to determine which of these functions are crucial to the maintenance of viral dependent transformation.